Cement and Concrete Technology in the 2000 sSecond International Symposium, 6-10 September, 2000. Istanbul, Turkey

Permeability Study of various Aggressive Mediain Repaired Concretes

Vassilia Kasselouri - RigopoulouNational Technical University, Dpt of Chemical Engineering,

Athens, Greece

Niki KouloumbiNational Technical University, Dpt of Chemical Engineering,

Athens, Greece

Vagelis PapadakisTitan Cement Company S.A.,

Elefsis, Greece

Stratos SioutisNational Technical University, Dpt of Chemical Engineering,

Athens, Greece

The aim of the present experimental work is the assessment of the permeability of plainconcrete as well as of repaired one by aggressive media such as chloride ions, water,nitrogen and carbon dioxide. Cementitious concrete repair mixtures consisting of silicafume-Ca(OH)2 with small amounts of of cement, aiming at the improvement of settingtime, have been used. Permeation results have been interelated with those of XRD andSEM measurements of the repaired mixtures and the repaired specimens. As it isconcluded these mixtures after a 28-day hydration excibit higher values of both gas andwater permeability as well as of chloride penetration than those of plain concrete.Nevertheless, the repaired specimens show acceptable permeability and theircarbonation remains, in every case, close to that of plain concrete. As these repairmixtures create CSH products in a sufficient extent having good binding properties animprovement of their permeability characteristics could be expected, as the time passesthrough.

The high alkalinity of concrete causes the formation of saturated hydrated iron oxidepassivating films on reinforcing steel, thus providing good protection against corrosion.

Nevertheless, corrosion remains the most common cause of concrete deterioration, asfinally the formation of rust exerts significant tensile forces within the concreteprovoking cracking of the concrete cover [1,2].

Passivation can be destroyed either by a decrease in the pH value (pH<9) due tocarbonation or by chloride ion penetration [3,4].This is of great importance both in marine structures and reinforced concrete roadbridges. Carbonation, being a rather slow process, induces steel bar corrosion only inconstructions with very thin concrete cover, high porosity, shrinkage cracks, and so forth[5]. Pores continuity and pore distribution as well as composition of the pore electrolytesolution determines chloride conduction mechanism in the concrete [6]. The steel barscorrosion onset, the time interval before the attack starts and the rate at which itproceeds are dependent on factors influenced by the chemistry of cement, mineraladditions, water binder ratio, curing and environmental conditions [7-10]. Among thempermeability of the concrete is recognized as a factor of great importance.

Since intrinsic permeability reflects the properties of the conductive medium alonegenerally it is independent of the permeant used provided there is no chemicalinteraction between conductive medium and permeant [11].The situation is different, when concrete is the conductive medium. The intrinsicpermeability is found to always be higher when nitrogen is used, compared to the samemeasured with water as permeant, independent of the age of concrete, type of cementand aggregates and water-cement ratio. This behaviour has been observed by manyinvestigations [12-14].

Although concrete possesses unique durability properties, after an incubation periodcorrosion is triggered, imposing an increasing need for concrete maintenance and repair.As it well known, there is a wide range of cementitious repair mortars based on cementand components similar to those of concrete. It is normally impossible to getinformation on the exact composition of a repair mortar or on many of the componentsused in it and consequently on their properties including that of the intrinsicpermeability they show with various permeants [15-17].

Previous studies have shown that cementerious concrete repair mixtures consisting ofSilica Fume - Ca(OH)2 with or without portland cement additions, form CSHcompounds with good binding properties resulting to low corrosion rate of rebars of therepaired cement mortars [18].

In the present work the permeability of plain concrete as well as of repaired one with theabove mentioned repair mixtures is examined. In all cases H20, N2, CO2 (from theenvironment) and cr are used as permeants. These results are interrelated with those ofXRD and SEM measurements of the repaired mixtures and the repaired specimens.

The specimens were constructed in this work using an Ordinary Portland Cement (145B512:1996) of 330 Kg/m3, limestone aggregates of 16mm maximum gradation andpotable water. The w/c ratio was equal to 0.64 and the cement/aggregate ratio to 0.18.

The gradation of aggregates is given in Table 1. The density of the fresh concrete was2.375 Kg/m3 and the slump 10 em.

% Passed from102235546881100

Size fraction (mm)0.250.50

1.0

2.04.06.016.0

The repair mixtures consisted of densified Silica Fume of Norwegian ongm (SF)(amorphous Si02 >93% wt) and commercial calcium hydroxide. The calcium hydroxidewas in slurry form containing 50% wt water. Two mixtures of the above materials havebeen used. The first one had a wt % composition of SF: 20.7 - Ca(OH)2: 76.3 - OPC: 3.0and the second one of SF: 20.0 - Ca(OH)2: 74.0 - OPC: 6.0. The small amounts ofcement have been added aiming at the improvement of the setting time of the SF -Ca(OH)2 mixtures.

The specimens were cast in cylindrical moulds having 10cm diameter and 5cm height.Two types of specimens were used: a) reference specimens of plain concrete and repairmaterials respectively and b) specimens consisted of plain concrete (4 cm height) and ofrepair mixture (1 cm) cust one on the other. In Table 2 the composition and the codenames of the specimens tested are shown. After custing the moulds were stored atambient temperature for 24h. Thereafter the specimens were demolded and stored incuring room (RR = 95+-5%, T = 20+-2°C) for 28 days.

Code namesABCCACB

CompositionSF-Ca(OH)2 + 3% OPCSF-Ca(OH)2 + 6% OPC

OPCOPC/AOPC/B

The permeability was measurement in a modified triaxial cell for 100mm diametersamples according to the procedure described in previous works [11,19]. The circuitsused for water and nitrogen permeation are shown schematically in Figure 1.

The chloride penetration resistance of concrete and repair materials was determined byapplying a potential of 60V DC and measuring the charge passed through the specimen,according to the AASHTO T277 rapid test method. The tested cores were slices 51 mmthick, cut from the concrete and repair specimens and coated with watertight tape ontheir cylindrical surface.

The carbonation depth of specimens exposed in atmosphere at ambient conditions wasdetermined by the phenolopthalein method recommended by RILEM (RIlEM CPC-I8)on broken concrete pieces at the interface concrete-repair mixture.

The study of the hydration products of the repair mixtures at predetermined ages by X-Ray Diffraction was performed by a D5000 SIEMENS Diffractometer.

For the study by Scanning Electron Microscopy (SEM), a ZEOL 6100 ElectronMicroscope has been used.

The results concerning the gas permeability of all types of specimens tested are given inFigure 2. As it is shown the gas permeability of the plain concrete is that of a cornmonone. The repair mixture containing 3% cement exhibits the highest values of gaspermeability, while the mixture containing 6% cement displays an improved behaviour.These results could be attributed both to the porous structure of the repair mixtures atthe age of 28-day hydration compared to that of the plain concrete and to thecontribution of the various cement contents of these specimens.

18NE 16(J).•..

14W0

12.•..~>. 10::::c 8nl<IlE 6•..<Il

40-tIlnl 2C)

0A

Figure 2: Gas permeability of 28-day hydrated specimens

When the plain concrete was covered by each of the repair mixtures the gas permeabilityvalues remained higher than those of plain concrete depending on the percentage of thecement content as well as on the height of the repair cover.Figure 3 represents the results of water permeability tests. It is clear that the behaviourof all types of specimens is slightly improved in comparison with that concerning thegas permeability. The concrete specimens covered with repair mixture containing 6%cement show water permeability value very close to that of plain concrete.

N 2.5

E(0.•.. 2,w0.•..~ 1.5~:craQl

E•..Ql 0.50..•..Ql.•..ra

03:A B C CA CB

Type of specimens

Figure 3: Water permeability of 28-day hydrated specimens

As can be seen in Table 3 the penetration of cWoride ions is high in the samples made ofrepair mixtures. These values remain high but closer to that of plain concrete whenconcrete specimens are covered by each of the repair mixtures.

SpecimensC-kBC

CACB

Coulomb3850

5000 - 70004000 - 60004500 - 60004000 - 5000

Permeability classMediumHighHighHighHigh

The penetration of CO2 has been estimated by measuring the carbonation depth onbroken pieces of the specimens at the interface concrete - repair mixture. The resultsafter 3 and 12 months of exposure at ambient conditions are presented in Figure 4. As isis shown after the first months of exposure, in all cases the carbonation depth changes alittle with time and does not reach the common thickness of the cement cover of thereinforcements of a structure.

E8

E..l: 6-c.Q)

't:lc: 40:;;C'tlc:02.c•...

C'tlU

~

cIl'.fJCA

DCB

Additionally, the carbonation depth of repaired specimens remains always very close tothat of plain concrete specimens. This could be attributed to the creation of a structure,at the interface concrete - repair mixture, similar to that of the plain concrete. Thissuggestion has been justified by XRD measurements. In Figure 5 the XRD pattern of therepair mixtures after a 28 days hydration is presented.

Figure 5: XRD patterns of hydrated repair mixtures after a 28 days hydrationa. SF-Ca(OH)2 - 3% wt OPC mixtureb. SF-Ca(OH)2 - 6% wt OPC mixture

In all cases the main compounds observed are Ca(OH)2 in the form of portlandite (18.1,34.2, 47.5° 28), small amount of CaC03 (29.4° 28) due to carbonation of Ca(OH)2 andcalcium silicate hydrates in the form of tobermorite (11° 28). As it is shown theformation of calcium silicate hydrates in the form of tobermorite, llA and 14A, as wellas in the form of2CaSi03' 3H20 (Riverseidite 5.5, 7.0, 12.0,28.7,29.8,31.0° 28) can beobserved from the first ages.

The binding properties between the main concrete mass of the plain specimens and therepair mixtures has been investigated by Scanning Electron Microscopy of the hydratedrepair material, SF-Ca(OH)2' In the figure 6 a grain of partly hydrated silica fume isshown. The microanalysis gave as result 52.39% CaO and 46.54% Si02 (CaO/Si02 ==1.26). In the centre of the grain's surface a cover of Ca(OH)2 is observed (75.92% CaOand 22.04% Si02 ; CaO/Si02 == 3.78).

Additionally, SEM analysis shows that at the interface between concrete and the repairmaterial sufficient formation of CSH can be observed from the first months of hydration(Figure 7). The form of these crystals is more similar to that of the hydrated cement,resulting in good binding properties.

Figure 7: SEM of interface between concrete and SF-Ca(OH)2 repair material after 2months of hydration (Mx5000)

1. The gas permeability of the pure repaired mixtures is higher than that of the plainconcrete, so far. The 6% cement addition exhibits significant improvementcomparing to 3% cement addition. In the case of the repaired specimens acceptablegas permeability has been observed.

2. The above conclusions characterize also the water permeability of the testedspecimens. An improvement of both gas and water permeability could be expected,as the time passes through, due to the progress of the hydration process of the repairmixtures.

3. The repaired specimens show high values of chloride ions penetration.4. The carbonation depth of the repaired specimens remains always close to that of theplain concrete ones.

5. Calcium silicate hydrates are formed in appropriate extent, creating good bindingproperties to the repair mixtures.

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